Abstract: An installation system for use with a window assembly with a nailing fin in a rough opening of a wall, the rough opening including a sill. The system includes a sill drain mat including a barrier wedge with a front edge, a rear edge, a bottom surface and a top surface. A fiber layer includes a top surface and a bottom surface and an adhesive layer disposed on the bottom surface of the fiber layer. A first portion of the fiber layer is adhered to the top surface of the barrier wedge and extends inwardly, the first portion of the fiber layer including a sealing strip. A second portion of the fiber layer extends outwardly from the front edge and the bottom surface of the barrier wedge forms a mating surface with a sill of the rough opening. A gasket includes a first surface attached to an inner surface of the nailing fin, a second surface received adjacent an outer surface of the wall, and an outer edge comprising a protruding end extending along a length of the gasket.